The present invention relates generally to an adaptive equalizer system and, more particularly, to an adaptive equalizer system for quadrature amplitudemodulated (QAM) waves for use in a digital radio transmission system.
A digital radio transmission system is typically susceptible to channel deterioration or momentary disconnection due to waveform distortion via multipath fading or the like occurring on the transmission path. These deficiencies can be surmounted by the use of an adaptive equalizer (hereinafter referred to as an equalizer) such as a transversal equalizer or a decision-feedback equalizer.
The first requirement for effective equalizer operation stems from the need, when the tap coefficient control loop (hereinafter referred to as the control loop) becomes asynchronized, for setting value of the variable tap coefficient control at an initial level such that this asynchronization can be eliminated. This action is commonly referred to as resetting. The resetting function improves the synchronizing performance of the equalizer and thereby maintains a stable synchronous state. A know method of realizing this resetting function is by providing an output of a voltage generator at the variable tap gain circuit of each tap in response to a signal of frame asynchronization or the like. For details on this method, reference is made to Japanese Patent Application Disclosure No. 49-2416 (or the corresponding West German Pat. No. 2,319,807. This method presents the disadvantage of requiring, apart from the equalizer, the power voltage generator for setting initial level as well as a relay for switching connections.
The second requirement for efficient equalizer operation, relative complexity or simplicity of structure, largely depends on the equalizing algorithm for the equalizer as well as the arrangement of the transversal filter section of the equalizer in the intermediate frequency (IF) band or in the baseband.
Equalizing algorithms involving relatively simplified circuitry, and as conventionally used in such equalizer applications, include the zero forcing (ZF)method and the modified zero forcing (MZF) method. Both methods, employing digital signal processing techniques, can be readily realized by using such simple circuits as logic circuits or shift registers. For details on these techniques, reference is made to D. Hirsch, "A Simple Adaptive Equalizer for Efficient Data Transmission", IEEE TRANSACTIONS ON COMMUNICATION TECHNOLOGY, Vol. COM-18, No. 1, pp. 5-12, February 1970.
Regarding the operational frequency band of the transversal filter section, since the input signals are quadrature amplitude-modulated waves, a transversal filter arranged in the IF band presents the advantage of containing the orthogonal and quadrature components of the modulated wave in a single signal. By such an arrangement no more than 2N.sub.1 variable attenuator circuits for providing variable tap coefficients are needed (the number of taps being represented by N.sub.1, a positive integer). A transversal filter in the baseband, on the other hand, would require 2N.sub.2 to 4N.sub.2 variable attenuator circuits for processing the orthogonal component of the input signal, (N.sub.2, a positive integer, representing the number of taps) thus involving complicated circuitry and corresponding is troublesome adjustment capabilities.
The simply structured transversal filter in the IF band, where the input carrier frequency is equal to an integral multiple of the modulation rate, readily permits the usage of a 2F equalizing algorithm. The number of phase rotations against the carrier wave in the signal delay circuit, which is selected to be equal to the inverse number of the modulation rate, is equal to an integral multiple of 2.pi. as are phase differences between the taps. However, when the input carrier frequency is unequal to any integral multiple of the modulation rate, phase differences between the taps are unequal and accordingly the filter is rendered incapable of utilizing the 2F algorithm.